1. Field of the Invention
The present invention generally relates to a Small Computer System Interface (SCSI), and more particularly for an improved SCSI driver circuit configured in an H-Driver embodiment adapted for varying amplitudes and rise times.
2. Description of the Related Art
SCSI circuits are generally known in the art. An SCSI circuit is an independent input/output bus which allows a variety of peripheral devices to be connected to a personal computer system. The SCSI was initially developed to provide a disk drive interface that supported logical addressing of data rather than the more prevalent physical addressing. Additionally, the SCSI circuit was developed to transfer information in a parallel, byte-wise fashion instead of serially, thus, ending certain compatibility difficulties associated with developing new disk drive technologies. The electrical characteristics and signal protocols of the SCSI circuit were developed in such a manner that the requirements of various peripheral devices could be accommodated with relative ease and flexibility. In particular, the SCSI protocol defines a number of commands which are available for accessing and querying a particular peripheral device regarding the parameter set required for the device to operate correctly. This particular feature of the SCSI makes it possible for a system designer to write a software device driver program for a generalized peripheral device, without regard to device specific parameter set details.
Differential SCSI busses are terminated at each end, and have SCSI devices such as hard disk drives attached along the length of the bus, with a minimum spacing of 4 inches. These devices are stubs which are unterminated. The stubs load the bus with a lumped capacitance from each device""s SCSI input/output (IO) driver, and effectively reduce the rise time at the far-end receiver, potentially causing timing errors. Additionally, backplanes are notorious for causing timing problems due to crosstalk, poor impedance control, and device spacing violations.
Adjustable driver rise times allow the user to optimize the driver for different bus configurations, and help minimize signal cross talk from faster than required edge rates. SCSI busses can also have a large attenuation loss due to series resistance in the cable and backplane. The loss can be as high as 6 db. Adjustable driver amplitudes allow the user to optimize the driver amplitude for the attenuation loss but also minimize signal cross talk from larger-than-required amplitudes. It is an important feature of these systems that the driver""s rise time and amplitude are stable across the chip process and power supply, so chips from different process lots are interchangeable.
The optimum driver output signal waveform is linear. Moreover, having a constant rate of change between states enables the far-end receiver to generate a digital pulse with minimal duty cycle distortion. The slew rate is the time rate of change of the rising or falling edge of a signal. If there is a reduction in the slew rate after the differential voltage zero crossing, what results is a timing skew due to the receiver""s finite gain. A linear shape, or constant slew rate, also enhances receiver noise immunity because the signal transits the critical +/xe2x88x92100 mv window at a constant rate. If the slew rate reduces during some portion of the rise or fall, then noise has a longer opportunity during that time to influence the receiver. Therefore, there is a need for a novel circuit design, which allows for independent control of both the amplitude and rise time for an SCSI differential driver.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional SCSI circuits, the present invention has been devised, and it is an object of the present invention to provide a novel design for an SCSI circuit technique which allows for independent control of driver slew rate and amplitude with a linear shaped driver output voltage. The technique uses the silicon area on the integrated circuit chip efficiently, which is at a premium in the IO pad region. Moreover, the circuit has excellent process and power supply tolerances.
In order to attain the object(s) suggested above, there is provided, according to one aspect of the invention, an SCSI driver circuit comprising a symmetrical H-Driver having at least four predrive controls. The SCSI driver circuit has a predrive control circuit that includes an amplitude current digital-to-analog converter, a rise time current digital-to-analog converter, and an operational amplifier. The predrive control circuit is coupled to one of the predrive controls for varying the amplitude and rise time.
The amplitude current digital-to-analog converter and the rise time current digital-to-analog converter are independently controlled. The amplitude current digital-to-analog converter alters voltage on the operational amplifier to control an amplitude of signal output from the predrive control circuit. The rise time current digital-to-analog converter alters current on the operational amplifier to control a rate at which the signal output reaches the amplitude.
Each predrive control circuit further comprises a plurality of transistors and at least one diode configured to allow for the independent control of the amplitude and rise time, wherein the predrive control circuit yields a linear waveform output.